PROJECT SUMMARY Poly(ethylene glycol) (PEG) based hydrogels are promising and versatile materials for the delivery of protein and cellular therapeutics for bone tissue engineering. However, concerns over PEG?s immunogenicity have emerged in recent years, and it has been estimated that as much as 25% of the population has antibodies against PEG due to exposure via pharmaceuticals, cosmetics, and other PEG-containing products. While the anti-PEG response has been found to reduce the efficacy of intravenously administered PEGylated drugs, its impact on the efficacy of regenerative therapeutics delivered from PEG hydrogels has not previously been studied and is unknown. The objective of this project is to address this knowledge gap through in vivo experimentation comparing animals conditioned to mount an anti-PEG response to immunologically nave controls. Aim 1 consists of a subcutaneous implantation study in mice, which will elucidate the effects of the anti-PEG immune reaction on the fundamental biological response to PEG hydrogels. Aim 2 involves a bone tissue engineering study in which the impact of the anti-PEG response on hydrogel-mediated delivery of bone morphogenetic protein-2 (BMP-2) will be tested in a murine calvarial defect model. Quantitative measurements of bone healing compared to control animals will be used to determine whether bone healing is negatively affected. The results of this exploratory project will either alleviate concerns over PEG immunogenicity for tissue engineering or motivate future investigations on strategies to mitigate its effects.